Mechanistic Insights into Chromium Reduction by Indigenous Soil Bacterial Consortia from Industrial Sites of Raipur, Chhattisgarh, India

Abstract

Industrial soil contamination by chromium (Cr) is an important environmental and public health concern because hexavalent chromium [Cr(VI)] is toxic, mobile, and carcinogenic. This study investigated the mechanisms involved in reducing toxic Cr(VI) to the less toxic Cr(III) form using an indigenous bacterial consortium isolated from chromium-contaminated industrial soils in Raipur, Chhattisgarh, India. Surface-soil samples from the Urla Industrial Area, Siltara Growth Centre, and Birgaon Industrial Estate contained 567.4 ± 26.35, 893.6 ± 31.72, and 347.2 ± 18.40 mg kg⁻¹ total chromium, respectively. Of 124 bacterial isolates, 18 exhibited Cr(VI)-reducing ability. Under optimised conditions (pH 7.0, 32 °C, and 2% glucose), a consortium comprising Bacillus cereus RC-7, Pseudomonas putida RC-11, and Lysinibacillus sphaericus RC-14 achieved 97.4 ± 1.2% reduction of 100 mg L⁻¹ Cr(VI) within 96 h. Enzyme assays showed increased chromate reductase activity and associated electron-transfer responses during Cr(VI) reduction. Biosorption data fitted the Langmuir (R² = 0.981) and Freundlich (R² = 0.954) models, indicating effective surface interactions. SEM-EDX and FTIR analyses indicated Cr(III) precipitation and chromium binding on bacterial cell surfaces, whereas RT-qPCR showed upregulation of the chrR and nfsA genes under chromium stress. These findings clarify the biochemical, molecular, and surface-interaction mechanisms associated with Cr(VI) detoxification and support further evaluation of the consortium for chromium-contaminated soils and wastewater.